The present invention relates to a variable displacement oil pump adapted to supply working fluid as a hydraulic pressure source.
U.S. Pat. No. 7,794,217 discloses a previously-proposed variable displacement oil pump which is applied to an internal combustion engine for an automobile.
The variable displacement pump disclosed in this patent is a vane-type variable-displacement oil pump. In this technique, an eccentricity amount of a cam ring is controlled in two stages according to rotational speed of the engine, on the basis of spring force of a spring and biasing force of a discharge pressure introduced into two control oil chambers which are separately formed between a pump housing and the cam ring. The spring is provided to bias the cam ring in a direction (hereinafter referred to as “eccentric direction”) that increases the eccentricity amount of the cam ring relative to a rotation center of a rotor. The discharge pressure introduced into the two control oil chambers acts to bias the cam ring in a concentric direction (i.e., opposite to the eccentric direction) against the spring force of the spring. Accordingly, the oil pump can supply oil to a plurality of devices which desire discharge-pressure values different from each other.
Specifically, when the engine rotational speed rises, the discharge pressure is introduced into one of the two control oil chambers. Then, when the discharge pressure reaches a first predetermined oil-pressure value which is a first equilibrium pressure, the cam ring moves somewhat in the concentric direction against the spring force of the spring. Then, if the engine rotational speed further rises, the discharge pressure is introduced also into another of the two control oil chambers in addition to the one of the two control oil chambers. Then, when the discharge pressure reaches a second predetermined oil-pressure value which is a second equilibrium pressure, the cam ring moves further in the concentric direction against the spring force of the spring.